1. Field of the Invention
The present invention relates to an image processing unit that applies an anti-aliasing processing to both an image signal and a key signal, when synthesizing a foreground image and a background image by using the key signal, and a method of the same.
2. Description of the Related Art
When one visual image (foreground image) is synthesized with another visual image (background image) in such a manner as the former hollows out the latter with an arbitrary shape, a key signal is used which represents a composite rate of a picture element of an image against a picture element of another image (hereafter, `picture element` is abbreviated by `pixel`). Generally, the key signal is given a value such that a part to be erased as the background is made transparent and a part not to be erased is made opaque by extracting a blue-back (background color) from an input image, or the like.
When a synthesized image in which the foreground image is synthesized with the background image by using this key signal is presented on a raster scanning color display, for example, a stepped aliasing can obviously be observed by some chance especially on a slant line or a periphery of the image plane. This is a phenomenon called the aliasing error resulting from an insufficiency of the number of pixels, which will invite a deterioration of the quality of a synthesized image. Accordingly, the anti-aliasing processing is considered to be necessary which makes the edge parts of the aliasing obscure not to be obvious.
However, only executing a filter operation directly to the key signal for the sake of the anti-aliasing processing will spread the key values on the background area that should originally be made transparent, and there is a possibility that the background color can be observed while synthesizing video signals.
That is, when a simple three-tap filter operation ##EQU1##
is applied to an input key signal value Ki, for example, as shown in FIG. 13(a) (drawn as i=0-9), the transparency is degraded as to the pixels of i=1, 8 to be originally the background area, as shown in FIG. 13(b).
The anti-aliasing processing becomes necessary also to the foreground image signal to be synthesized. However, only applying a simple filter operation to the foreground image will mix the background color on the border area of the synthesized image, which should originally be removed by the key and should not be observed, which poses a problem.
That is, when a simple three-tap filter operation ##EQU2##
is applied to an input image signal value Pi, for example, as shown in FIG. 14(a) (drawn as i=0-9), the foreground color is mixed in i=1, 8 to be originally the background area, and the background color is mixed in i=2, 7 to be the foreground color, as shown in FIG. 14(b).